Liquid measuring condenser



Aug. 7, 1951 LE ROY A. GRIFFITH Filed April 15, 1946 LIQUID MEASURING CONDENSER Patented Aug. 7, 1951 UNITED STATES PATENT OFFICE LIQUID MEASURING CONDENSER Le Roy A. Griffith, Minneapolis, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application April 15, 1946, Serial No. 662,180

Claims. (Cl. 175-415) This invention relates to the field of instruments for the measurement of liquid fuel, and more particularly to the sensing or pickup units of such systems, especially units in which variation in the amount of fuel causes change in the capacitance of a measuring condenser.

In the normal operation of vehicles using liquid fuel, for example aircraft, a certain amount of water gradually appears in the gasoline tank, derived mostly by condensation from atmospheric air drawn into the tank to replace fuel used during flight. Condensation also takes place in the tanks of aircraft which are grounded for extended periods. Being heavier than gasoline, the water naturally takes its place at the bottom of the tank, and in the course of time a suflicient quantity of water may collect so that its level reaches to the sensing units of the fuel measuring system. Such water is of course drained out of the tanks at necessary intervals, but if the level of water has reached the sensing units, an aqueous film remains on them after the tank is drained. This is particularly undesirable in condenser type pickup units, since the resistivity of water is widely at variance with that of gasoline, and the instrument therefore gives readings of fuel quantity which are inaccurate to an extent determined principally by the ohmic losses in the pickup due to the water film.

It is an object of this invention to provide a capacity pickup for a liquid fuel measuring system in which the effect on the indication of the system of ohmic losses in the pickup unit is minimized. I

It is another object of the invention to provide a capacity pickup unit comprising concentric cylindrical electrodes mounted on and insulated from a support member, the electrodes and the member being in direct contact with the fuel.

Various other objects, advantages and features of novelty which characterize my invention are pointed out with particularity in the claims annexed hereto and forming a part hereof. However, for a better understanding of the invention, its advantages, and objects obtained by its use, reference should be had to the subjoined drawing, which forms a further part hereof, and to the accompanying descriptive matter, in which I have illustrated and described a preferred embodiment of my invention. In the drawing,

Figure 1 is a sectional elevation of an embodiment of a pickup unit according to the invention,

Figure 2 is a, schematic wiring diagram showing the unit in use in a complete measuring sys- Figure 3 is a disclosure of thenormal arrangement in prior art devices, and

Figure 4 is a top view of the screw head and bushing shown in Figure 1.

Referring first to Figures -1 and 2, there is shown a sensing unit In comprising a cylindrical housing H having a closure l2 at one end and a flange l3 at the other end. Housing I l is mounted by conventional threaded means ii on circumferentially spaced bosses l9 projecting from a base member I 4. Flange I3 is fastened to housing H by any suitable means: Figure 1 shows a spun interlock l1, and additional rigidity may be given to the structure by spot welding if desired.

End closure I2 is fastened to housing II by conventional threaded means 2|, and is shown to comprise 8, central boss 22 connected by a flange 23 with an outer rin 24: radial apertures are formed in ring 24. A hollow bushing 30 is tightly pressed into a central aperture in boss 22, and is freely traversed by a screw 21. As is shown in Figure 4 the bushing is slotted transversely at one end to provide a non-circular recess bordered on two sides by arcuate sectors of the bushing, and the head 26 of screw 21 is shaped to be received within the opening so formed. The noncircular shape of th aperture prevents the screw from turning independently of bushing 30 and hence of closure l2, but permits axial displacement of the screw. A plurality of further openings 3| are provided in boss 22, each of which receives a spiral spring 32, for a purpose presently to be described.

Member I4 is provided with a centrally disposed aperture 33 of circular cross-section, and an eccentrically disposed aperture 34, also of circular cross-section extending in the direction of housing II. The member is also provided with an annular recess 35 partially occupied by an annular insulating member 36. Member 36 in turn is annularly recessed to receive the flange 31 of a first cylindrical electrode 40 which fits within a first Member 4I has an out-turned flange portion which is engaged between flange I3 and insulating member 36 of member I4, so that electrode 48 is held firmly against axial movement with respect to member I4. A stud 44 secured to flange 31 traverses member 36 and the eccentrically disposed opening 34 in member I4, being spaced therefrom by a suitable insulating member 49 and a packing gland 48, so that by tightening a first nut I6, gland 48 is compressed between mem bers 49 and 36, providin a fluid tight seal for aperture 34 and fastening electrode 48 rigidly to member I4. A second nut 28 secures to stud 44 a lug 46 to which electrical connection is made by a wire 41 from a conventional electrical connector 58. Member 36 is provided with apertures 59, arranged to coincide with the spaces between bosses I9, to admit the fluid to be measured into the space within the electrode.

A second electrical connector is joined by a conductor 52 with a lug 53 fastened by suitable means 54 to a second stud 55 which traverses the centrally disposed aperture 33 in member I4 through suitable insulating discs 56 and 51 and a packing gland '69. At its inner end stud 55 is enlarged to cooperate with a second cylindrical electrode 6|, to which it is shown as fastened by welding, and at its outer end stud 55 threadedly engages a nut 63, so that by tightening nut 63 gland 68 is compressed between members 51 and 56, and the whole assembly is clamped firmly to member I4.

The end of electrode 6| remote from member I4 is welded to a conically recessed disc 64, interfitting with a convexly conical insulating member 65 which threadedly engages with screw 21 previously recited, so that the outer ends of springs 32 engage the base of member 65. Thus, when the unit is assembled, member 65 engages recess 64, in resilient fashion, due to the compression of coil springs 32, the head 26 of machine screw 21 taking up a position intermediate the extremes of its possible travel axially of bushing 38. A plurality of radial openings 69 provide means of egress for air being replaced in the space between electrodes 48 and BI by fluid entering the space through apertures 59.

Housing I I is provided with openings 58 to prevent trapping of condensate or fuel between it and electrode 48.

The unit is completed by the provision of a cover plate 61 fastened to member I4 in watertight fashion by any suitable means such as gasket 18 and machine screws 1I and is mounted in the bottom of a container 68 so that electrodes 48 and SI are within the container, and so that member I4 seals the hole in the container through which connectors 58 and 5| and their housing project.

Since the invention in the present application resides in the sensing unit per se and in the manner in which it is placed into a circuit, the circuit of a complete system using the sensing unit shown in Figure 2 is given schematically, but omits many refinements known to those skilled in the art. In general the system is closely related to the subject matter of the copending application of Curtiss R. Schafer et al., Serial No. 575,168, filed January 29, 1945, and assigned to the assignee of the present application, but my invention is an improvement on the structure thereshown and embodies novel features of structure and arrangement to give improved function over the earlier arrangement.

In Figure 2, there is shown a, transformer 12 having a primary winding 13 and a secondary winding 14 having end terminals 93 and 94 and intermediate taps 99, I81 and I38. Primary winding 13 is energized with alternating current from a suitable source 16 through conductors 11 and 88 and conductors 8| and 82. The circuit is shown to include a first empty calibration potential divider 83 having a winding 84 and a slider 85, and energized from taps 99 and I81 of winding 19 of transformer 12 through conductors 86 and 81. There is also provided a second rebalancing potential divider 98 having a, winding 9I and a slider 92, winding 9I being energized from tap I38 and terminal 94 of secondary winding 14 of transformer 12 through conductor 15, conductor 18, and a full calibration variable resistor 18.

Potential dividers 83 and 98 comprise a portion of a balanceable network 95 which also includes unit I8, as a measuring capacitor, and a comparison capacitor 96. The output terminals of the network are indicated at 91 and 98, the former being grounded. Ground connections 88 and 89 and shielded conductor I88 serve to connect across the output terminals of the network a resistor I8I comprising the input resistance of an amplifier I82, the latter being shown as chergized from source 16 by conductors 11 and I83 and conductors 8| and I84. Amplifier I82 is also shown as energizing a motor I through a multiconductor cable I88.

Motor I85 is shown as actuating slider 92 of potential divider 98 through a suitable mechanical connection I88. The motor also actuates a movable index II8 across a fixed scale II9 by means of a suitable mechanical connection I28.

The mechanical structure of unit I8 as shown in Figure 2 has been somewhat generalized, and there have been superimposed thereon symbols indicative of the electrical function of the unit. Thus, the capacitance between electrodes 48 and GI has been indicated by condenser III and in addition to this capacitor there are shown a pair of resistors I12 and H3. The unit is connected with network by a shielded conductor H6 and by a limiting resistor II8 and an unshielded conductor II1: each of these conductors has capacity to ground and these capacitances are indicated by condensers I I4 and I I5.

Operation It will be apparent that, upon energizing the system shown in Figure 2 from source 16, alternating voltages of the same phase are supplied by transformer action between the upper and lower terminals of the windings of the potential dividers. A sensing circuit may be traced through the network from the terminal 93 of winding 14 through resistor II8, conductor II1, unit I8, shielded conductor II6, output terminal 98, shielded conductor I88, input resistor I8I of amplifier I82, ground connections 89 and 88, and output terminal 91 to slider 85. A comparison circuit may likewise be traced from slider 85 of potential divider 83 through output terminal 91, ground connections 88 and 89, input resistor I8I, shielded conductor I88, output terminal 98, and comparison condenser 96, to slider 92.

Load resistor I8I is common to both the sensing circuit and the comparison circuit, but it will be evident that the sensing current and the comparison current in input resistor I8I are degrees out of phase. When the sensing current and the comparison current are equal in magnitude, no potential difference is impressed on amplifler I82.

The voltage effective in the sensing circuit is determined by the position of slider 88, and the current through resistor I8I due to any particular voltage applied to the sensing circuit varies only with the impedance of unit I8, since the other elements in the circuit are constant. Simi' larly, the voltage effective in the comparison circuit is determined by the position of sliders 85 and 82 and the adjustment of resistor 18, the latter determining the current through winding SI and therefore the maximum voltage drop ob tainable along the winding. If the position of slider 85, the resistance of resistor I9, and the impedance of condenser 88 are all held constant, the current through resistor I8I due to the voltage applied to the comparison circuit varies only with movement of slider 82. Proper selection of circuit constants makes it possible for the sensing and comparison currents to be equal as previously described.

In originally setting up the instrument, the pickup is inserted into the tank in which it is to be used and fuel is admitted to the tank until it comes up to the lowest level from which fuel can usefully be drawn from the tank. Slider 85 is then adjusted until motor I85 comes to rest with slider 82 near the upper end of its travel and with pointer H8 at the empty graduation on scale II8. More fuel is now admitted, and the motor operates as fuel is being added until it stops in some position near the lower end of its travel when the tank is full. Resistance 18 is now varied to set pointer I I8 at the full? graduation on scale I I9.

When the tank is full, the dielectric between cylinders 48 and BI is almost entirely fluid, and the impedance of the pickup is minimum, allowing maximum flow of current in the sensing circuit. When the tank is empty the dielectric between cylinders 48 and 6| is almostentirely air, and the impedance of the pickup is maximum, allowing minimum flow of current in the sens- 1 8 circuit.

Suppose the tank is partly full, and the motor is at rest: the sensing and comparison currents are equal and opposite, and there is no voltage impressed on the amplifier. Now let a certain quantity of fuel be added to the container in which sensing unit I8 is installed. The fuel replaces air as the dielectric of a portion of the capacitor, thus increasing the average dielectric constant and the capacitance of the capacitor and decreasing its impedance. The magnitude of the current in the sensing circuit is thus increased so that it no longer equals that of the current in the comparison circuit, and a voltage in phase with the source appears between termlnals 81 and 88 of the network and is impressed on amplifier I82.

Amplifier I 82 may be any suitable motor control amplifier adapted to energize a motor for forward or reverse operation according as the phase of an input voltage is the same as or opposite to the phase of a standard voltage: such amplifiers are well known in the art, and amplifler I82 is therefore not shown in detail. The effect of impressing the unbalance voltage of the network, just described, on the amplifier is to energize motor I85 for operation in a first or "forward direction, displacing index II8 along scale II9 toward its full position, and at the same time moving slider 92 downwardly along winding 8|, and thus increasing the voltage be- 6 tween sliders 88 and 82. when this voltage is Just large enough to cause the current in the comparison circuit to equalthat in the sensing circuit, the voltage impressed on the amplifier disappears, and operation of motor I88 is interrupted.

If new fuel is withdrawn from the container by drainage or for use, the average dielectric constant of the sensing condenser decreases, thus decreasing its capacitance and increasing its impedance. The current in the comparison circuit now exceeds the current in the sensing circuit, and a voltage appears between terminals 81 and 88 of the network which is of the opposite phase to that appearing under the former condition. impressing this voltage on amplifier I82 results in operation of motor I88 in a second or "reverse direction which is opposite to that in which it formerly operated, moving pointer II8 along scale II8 towards its empty" position, and moving slider 82 upward along 8| to decrease the voltage applied to the comparison circuit and thus decrease the comparison current. When the two currents in resistor I82 are again equal, operation of motor I85 is again interrupted.

An understanding of the basic operation of the system having been reached, it is now possible to point out the manner in which my improved system, comprising the subject matter of this application, differs from systems previously disclosed. It will be observed by reference to Figure 2 that fluid between electrode 88 and electrode BI is in direct contact with member I4, which in turn is grounded. In prior art structures, the arrangement is such that not only are electrodes 88 and GI insulated from member I8, but the member is substantially completely overlaid with an insulating layer, so that the leakage path between the condenser plates comprises large areas of dielectric without any intermediate grounded conducting portions. The electrical functioning of this prior art structure is shown in simplified form in Figure 3, in which parts identical with parts shown in Figure 2 are given the same reference numeral primed. It will be seen that the principal difference between Figure 2 and Figure 3 is to be found in the relationship between the resistors associated with condenser III and the resistor associated with condenser III. These resistors represent the leakage losses in the condensers, and as is shown in Figure 3, the resistance I2I is effectively in parallel with the capacitor, while in Figure 2, the resistances H2 and H3 are in effect connected between the opposite plates of the capacitor and ground. It will be appreciated that so long as the dielectric of the capacitor has an extremely high ohmic impedance, as is generally the case with fluid fuel, the circuits are essentially identical, but when a relatively low impedance path such as that caused by a water film on the pickup parts is introduced between the plates of the capacitor, the effect on a circuit such as that shown in Figure 3 is to seriously alter the impedance in the sensing arm of the network, as well as shifting the phase of the current flowing therethrough, thus bringing about serious errors in the indication of the system.

Where the film is in electrical contact with the grounded member I4 of the pickup unit, however, the ohmic resistance is no longer in parallel with the capacitor, but comprises two portions as indicated by resistors H2 and H3. A study of the circuit shows the resistor H2 is essentially connected in parallel with the portion of winding 14 above slider 85. Accordingly, if the regulation of the transformer is good, variation in the magnitude of resistance H2 will have little effect on the impedance of the sensing arm of the network, and therefore will have little effect on the indication of the instrument.

In a similar fashion the resistance of resistor H3 is effectively in parallel with the input circult of the amplifier, rather than across the capacitance Ill. Within reasonable limits the only effect of this resistance is to slightly reduce the sensitivity of the amplifier, and its effect on the indication of the instrument is therefore much less than the effect of a corresponding resistance directly across the plate of capacitor I I I.

It will be appreciated that the sensing units in a fuel measuring system such as that here disclosed are located in the fuel tanks of the vehicle, generally in the wings of the craft, while the amplifier, indicator, etc., are located at some central position to give supervision of the fuel supply of the vehicle in coordination with other available data. This means that conductors H6 and Ill must be of considerable length, and this in turn increases the capacitance of each of these conductors with respect to ground as is suggested by capacitors H4 and H5 in Figure 2. It will be apparent however that, just as in the case of the resistors H2 and H3, capacitors I I4 and H5 respectively are effectively in parallel with the portion of winding 14 above slider 85 and with the input to amplifier I02, respectively, and therefore affect only the sensitivity of the instrument and not the actual unbalance of the network.

From the foregoing description, it will be apparent that I have greatly improved known fuel measuring systems by providing means for minimizing the affect on the indication of the system of the presence in the indicator of undesired fluid having conductivity characteristics which may difier widely from that of the fluid being measured.

Numerous objects and advantages of my invention have been set forth in the foregoing de scription, together 'with details of the structure and function of the invention, and the novel features thereof are pointed out in the appended claims. The disclosure, however, is illustrated only, and I may make changes in detail, especially in matters of shape, size, and arrangement of parts, within the principle of the invention, to the full extent indicated by the broad general meaning of the term in which the appended claims are expressed.

I claim as my invention:

1. A capacitor for insertion into an opening in the bottom of a container of dielectric fluid comprising in combination, an electrically conductive base member adapted to sealingly close said opening, a first condenser plate, first insulating means resting on said member and mounting said first plate above said member, a second condenser plate substantially enveloping said first plate, second insulating means resting on said member and spaced from said first insulating means and mounting said second plate above said member, said insulating means and said member cooperating to provide a surface leakage path between said plates which at all points includes a portion of said member, said capacitor being formed with a passage through a portion thereof to admit fluid into the space between said plates.

2. A capacitor for insertion into an opening in the bottom of a container of dielectric fluid com- 8 prising in combination, an electrically conductive base member adapted to sealingly close said opening, inner and outer concentric cylindrical condenser plates, an insulating disc mounting said inner plate on said member, an insulating annulus mounting said outer plate on said member so that a surface leakage path between said plates is provided which at all points includes a portion of said member, said capacitor being formed with a passage through a portion thereof to admit fluid into the space between said plates.

3. A tank unit for liquid fuel measuring equipment comprising in combination: an electrically conductive support member; a cylindrical housing mounted on said support member in electrical contact therewith to comprise a unit for fluid tight insertion through the bottom of a container of fuel; a first cylindrical electrode; first insulating means mounting said first electrode on said member; second insulating means supporting said first electrode in concentric alignment with said housing; a second cylindrical electrode; third insulating means mounting said second electrode on said member so as to provide a leakage path between said electrodes which at all points includes said first and third insulating means and a portion of said support member; insulating means resiliently supporting said second electrode in concentric alignment with said housing; means making electrical connection with said electrodes; said third insulating means having apertures and being spaced from said first insulating means in such a manner as to admit said fiuid into the space between said electrodes to contact said electrodes and said member and to replace air as the dielectric of a condenser including said electrodes.

4. A capacitor for insertion into an opening in the bottom of a container of dielectric fiuid comprising in combination, a closure member for attaching said capacitor to the container and adapted to sealingly close said opening, said closure member including an electrically conductive annular portion, av first condenser Plate, a second condenser plate substantially enveloping said first plate, and insulating means resting on said closure member and having spaced portions for supporting said first and second condenser plates above said closure member, said spaced portions of said insulating means forming the walls of a recess between said plates with said electrically conductive annular portion at the bottom of the recess to provide a surface leakage path between said plates which at all points includes a portion of said electrically conductive annular portion, said capacitor being formed with a passage through a portion thereof to admit fluid into the space between said plates.

5. A capacitor for insertion into an opening in the bottom of a container of dielectric fluid comprising in combination, an electrically conductive base member adapted to sealingly close said opening, a first condenser plate, first insulating means resting on said member and mounting said first plate above said member, said first condenser plate having a central extension extending through said first insulating means for clamping said first plate in position and for providing one electrical terminal, a second condenser plate substantially enveloping said first plate, and second insulating means resting on said member and-spaced from said first insulating means and mounting said second plate above said member, said second condenser plate having a plurality of fastening members extending there from thromh said second insulating means and one 01' which provides a second electrical terminai. said insulating means and said member cooperating to provide a surface leakage path between said plates which at all points includes a portion of said member, said capacitor being formed with a passage through a portion thereof to admit fluid into the space between said plates.

LE ROY A. GRIFFITH.

REFERENCES CITED The following references are of record in the file of this patent:

Number Number 10 UNITED STATES PATENTS Name Date Cleary Oct. 11, 1932 Wappler June 12, 1934 Freystedt Jan. 30, 1940 Dubilier Apr. 17, 1945 FOREIGN PATENTS Country Date Great Britain Jan. 22, 1936 

